December 1, 2005 14:33 WSPC/Guidelines-IJMPA03029

International Journal of Modern Physics A
Vol. 20, No. 29 (2005) 6852–6854
c World Scientific Publishing Company

Int. J. Mod. Phys. A 2005.20:6852-6854. Downloaded from www.worldscientific.com
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A NEW STUDY ON THE ENERGY SPECTRUM AND COMPOSITIONOF PRIMARY COSMIC RAY FLUX AT ENERGIES ∼ 1014 − 1016 EV
USING THE GRAPES-3 ARRAY AT OOTY

S. C. TONWAR,∗ S. K. GUPTA, D. K. MOHANTY, P. K. MOHANTY,K. SIVAPRASAD, B. V. SREEKANTAN,
Tata Institute of Fundamental Research, Homi Bhabha Marg, Colaba, Mumbai, MH-400005 India∗tonwar@tifr.res.in
Y. HAYASHI, N. ITO, S. KAWAKAMI, T. NONAKA, H. TANAKA and T. YOSHIKOSHI
Graduate School of Science, Osaka City University, Osaka, 558-8585 Japan
Received 25 October 2004
Data collected with the 217-detector air shower array and the 560 m 2 area tracking muon detector, being operated at Ooty in southern India by the India-Japan (Tata Institute-Osaka City University)
collaboration, GRAPES, have been analyzed to study the shape of the energy spectrum and the composition around the knee. It is shown that the muon multiplicity distribution, observed with the highly
modular muon detector, permits a relatively reliable measurement on the composition of primary flux
which then helps in a more accurate reconstruction of the energy spectrum from the observed shower
size spectrum. The highlights of the GRAPES array, the analysis procedure and the results are presented.
Keywords: Primary Cosmic Rays; Spectrum and Composition; Extensive Air Showers

1. Introduction
A signature of the physical process responsible for the knee in the primary cosmic rayenergy spectrum is being sought in the change in the composition of primary flux at PeV
energies. Due to severe practical constraints on the long duration flights with heavy payloads, studies in cosmic ray astrophysics at energies ≥ 100 TeV have to necessarily rely on
indirect measurement on various components of air showers carried out at mountain altitude, sea level or underground locations. It is well-known that the muon component in air
showers offers the highest sensitivity1 for studies on the nuclear composition of primarycosmic ray flux around the knee. The GRAPES-3 collaboration is attempting to measure
the primary composition from a study on the muon multiplicity distribution for showers
over a wide energy range using a very large, 560 m2 area, muon detector. The GRAPES-3
experiment is located at Ooty (11.4◦ N latitude, 76.7◦ E longitude and 2200 m altitude) insouthern India.
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A New Study on the Energy Spectrum and Composition of Primary Cosmic Ray Flux

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6m2m

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3210Fig. 1. Schematic layout for the 721 detector (opencircles ◦) GRAPES-3 shower array of which 217 detectors (filled circles •) are used for the present analysis. Each of the 16 squares represents a 35 m2 area
muon tracking detector with Eµ ≥ 1 GeV.

Fig. 2. Schematic of two muon detector modules ofa super-module, showing the 4 layers of proportionalcounters (marked by 0, 1, 2, 3) embedded within theconcrete blocks. Proportions are not preserved.

2. The GRAPES-3 Experiment
Figure 1 shows a schematic layout of the GRAPES-3 array.2 A total of 721 detectors areplanned to be installed over a period of time. Presently only the data from the inner 217
detectors are being used for studies on the energy spectra and the composition of primarycosmic ray flux. Figure 1 also shows 16 squares in the north-west region. Each of thesesquares represent a 4-layer tracking muon detector module. 3 Each layer consists of 58
proportional counters, each 6 m long with 10 × 10 cm2 cross-sectional area. To achieve anenergy threshold of 1 GeV for vertical muons, ∼ 550 g cm−2 thick absorber in the formof concrete blocks has been used. A cross-section of two adjacent muon detector modules
is shown schematically in Fig. 2. Four such modules, separated by a horizontal distance of
130 cm at the base, constitute a super-module.
3. Data Taking and Analysis
Datataking started with the GRAPES-3 array in 2001 and preliminary results are now available from data taken during 2001–02. In an attempt to have the lowest possible energy
threshold for the determination of the composition of the primary flux, the processing ofdata has been kept to a minimum as far as the reconstruction of showers is concernedfor estimating shower size and age. Instead, the parameter Psum has been used to group
showers which represents the sum of the number of particles observed in the inner 127
detectors in a shower. The multiplicity distributions for the muons observed in the 560 m 2
area muon detector have been plotted for showers with various Psum values. Fig. 3 shows
the typical distributions observed for 4 broad range of Psum values. These distributions
have been compared (Fig. 3) with the expectations from Monte Carlo simulations using
the CORSIKA code and the QGS model for hadron interactions. This comparison shows
the good sensitivity of the muon distribution to the atomic number of the primary particle.

December 1, 2005 14:33 WSPC/Guidelines-IJMPAInt. J. Mod. Phys. A 2005.20:6852-6854. Downloaded from www.worldscientific.com
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S. C. Tonwar et al.

(a) Psum < 30(b) Psum 3050(c) Psum 50100(d) Psum > 100

Fig. 3. A comparison of muon multiplicity distributions observed with the 560 m 2 area muon detector withresults obtained from Monte Carlo simulations for various primary particles.

4. Results and Conclusion
Based on detailed comparison of observations with the results from simulations and assuming the primary cosmic ray flux to be consisting of 5 nuclear groups (p, He, CNO, Si andFe), it has been possible to obtain the estimates for the flux of these nuclei in the primaryflux. The observed flux values at energies near 1014 eV agrees very well with measurementsfrom direct measurements4 with balloon-borne detectors. Also, the observed steepening in
the all-particle energy spectrum at energies around a few PeV confirms the presence of theknee, as reported by several other groups.5 The GRAPES-3 results on the variation of ln A
also agree well with the results presented by the KASCADE group,6 based on the muoncomponent.
With more statistics available from data collected during 2002–04, these measurements
will be extended well beyond the knee which may provide some clues to the origin of ultrahigh energy cosmic rays.
References
1. S. K. Gupta et al., Phys. Rev. D68, 052005 (2003).
2. S. K. Gupta et al., to appear in Nucl. Instrum. Meth. A (2004).
3. Y. Hayashi et al., submitted to Nucl. Instrum. Meth. A (2004).
4. J. P. Wefel et al., J. Phys. G: Nucl. Part. Phys. 29, 821, (2003).
5. J. R. Hörandel, Astropart. Phys. 19, 193 (2003).
6. T. Antoni et al., Astropart. Phys. 16, 245 (2002).